Quantum dot laser diode and method of manufacturing the same

ABSTRACT

Provided are a quantum dot laser diode and a method of manufacturing the same. The method of manufacturing a quantum dot laser diode includes the steps of: forming a grating structure layer including a plurality of gratings on a substrate; forming a first lattice-matched layer on the grating structure layer; forming at least one quantum dot layer having at least one quantum dot on the first lattice-matched layer; forming a second lattice-matched layer on the quantum dot layer; forming a cladding layer on the second lattice-matched layer; and forming an ohmic contact layer on the cladding layer. Consequently, it is possible to obtain high gain at a desired wavelength without affecting the uniformity of quantum dots, so that the characteristics of a laser diode can be improved.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to and the benefit of Korean PatentApplication Nos. 2005-118104, filed Dec. 6, 2005, and 2006-86028, filedSep. 7, 2006, the disclosures of which are incorporated herein byreference in their entirety.

BACKGROUND

1. Field of the Invention

The present invention relates to a distributed feedback quantum dotlaser diode and a method of manufacturing the same, and moreparticularly, to a quantum dot laser diode in which a grating structureis formed under a quantum dot and a method of manufacturing the same.

2. Discussion of Related Art

Recently, quantum dot laser diodes have been developed that use quantumdots grown by a self-assembly method in an active layer. Specifically,such quantum dot laser diodes have a broad-area structure, aridge-waveguide structure, a buried hetero-structure, a structure havingan externally patterned metal grating and so on.

However, according to recent research tendency of growing In(Ga)Asquantum dots on an InP substrate by the self-assembly method, it isrelatively hard to form uniform quantum dots in comparison with anIn(Ga)As quantum dot structure formed on a GaAs substrate. In addition,when quantum dots having relatively low uniformity are applied to alaser diode, gain that can be obtained at a desired lasing wavelength isreduced. In this reason, it is hard to obtain excellent devicecharacteristics. Therefore, current research is intended to provide amethod for enhancing the uniformity of quantum dots in a desiredwavelength region or improving device characteristics at a specificwavelength in order to improve device characteristics upon manufacturinga laser diode using In(Ga)As quantum dots as an active layer on an InPsubstrate.

SUMMARY OF THE INVENTION

The present invention is directed to a quantum dot laser diode having agrating structure formed on a substrate to provide high gain in adesired wavelength region without affecting the uniformity of quantumdots, and a method of manufacturing the same.

One aspect of the present invention provides a method of manufacturing aquantum dot laser diode, comprising the steps of: forming a gratingstructure layer including a plurality of gratings on a substrate;forming a first lattice-matched layer on the grating structure layer;forming at least one quantum dot layer having at least one quantum doton the first lattice-matched layer; forming a second lattice-matchedlayer on the quantum dot layer; forming a cladding layer on the secondlattice-matched layer; and forming an ohmic contact layer on thecladding layer.

In the step of forming at least one quantum dot layer, one of ametal-organic chemical vapor deposition (MOCVD) method, a molecular beamepitaxy (MBE) method, an atomic layer epitaxy (ALE) and a chemical beamepitaxy (CBE) method may be used. The quantum dot may be formed ofIn(Ga)As. In the steps of forming the first and second lattice-matchedlayers, the first and second lattice-matched layers may be formed of agroup III-V compound.

In the steps of forming the first and second lattice-matched layers, thefirst and second lattice-matched layers may be formed to have a separateconfinement hetero-structure (SCH) having a step index (SPIN)-shapedwaveguide. When the first and second lattice-matched layers are formedin the SPIN-shaped SCH structure, a quantum well may be inserted intothe SPIN SCH structure and formed to symmetrically or asymmetricallysurround the quantum dot.

In the steps of forming the first and second lattice-matched layers, thefirst and second lattice-matched layers may be formed to have an SCHstructure having a graded index (GRIN)-shaped waveguide. When the firstand second lattice-matched layers are formed in the GRIN-shaped SCHstructure, a quantum well may be inserted into the GRIN SCH structureand formed to symmetrically or asymmetrically surround the quantum dot.

Another aspect of the present invention provides a quantum dot laserdiode comprising: a grating structure layer formed on a substrate andincluding a plurality of gratings; a first lattice-matched layer formedon the grating structure layer; at least one quantum dot layer formed onthe first lattice-matched layer and including at least one quantum dot;a second lattice-matched layer formed on the quantum dot layer; acladding layer formed on the second lattice-matched layer; and an ohmiccontact layer formed on the cladding layer.

The substrate may be an InP substrate or a GaAs substrate. The quantumdot may be formed of In(Ga)As. The first and second lattice-matchedlayers may be formed of a group III-V compound. The first and secondlattice-matched layers may have an SCH structure having a SPIN orGRIN-shaped waveguide.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features and advantages of the present inventionwill become more apparent to those of ordinary skill in the art bydescribing in detail preferred embodiments thereof with reference to theattached drawings in which:

FIGS. 1A to 1F are manufacturing process diagrams illustrating steps formanufacturing a quantum dot laser diode according to an exemplaryembodiment of the present invention;

FIG. 2A is a cross-sectional image of a quantum dot laser diode taken bya scanning electron microscope (SEM) according to an exemplaryembodiment of the present invention;

FIG. 2B is an image of a surface of a quantum dot laser diode taken byan optical microscope according to an exemplary embodiment of thepresent invention;

FIG. 3 illustrates light emission spectrum graph (i) of a quantum dotlaser diode including a grating structure layer and light emissionspectrum graph (ii) of a quantum dot laser diode not including a gratingstructure layer according to an exemplary embodiment of the presentinvention;

FIG. 4A is a graph showing optical power as a function of appliedcurrent measured under a continuous wave condition at room temperaturein a quantum dot laser diode manufactured according to an exemplaryembodiment of the present invention; and

FIG. 4B is a graph showing a spectrum obtained by applying apredetermined current to a quantum dot laser diode manufacturedaccording to an exemplary embodiment of the present invention.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Hereinafter, exemplary embodiments of the present invention will bedescribed in detail. However, the present invention is not limited tothe embodiments disclosed below, but can be implemented in variousforms. Therefore, the following embodiments are described in order forthis disclosure to be complete and enabling to those of ordinary skillin the art.

FIGS. 1A to 1F are manufacturing process diagrams illustrating steps formanufacturing a quantum dot laser diode according to an exemplaryembodiment of the present invention.

Referring to FIGS. 1A to 1F, in order to manufacture a quantum dot laserdiode 100, a substrate 110 is prepared first, and a grating structurelayer 120 including a plurality of gratings 130 is formed on thesubstrate 110 (see FIG. 1A). In general, the gratings 130 can be usedfor injecting photons and leading the stimulated emission. In thisembodiment, the layer including the gratings 130 is called the gratingstructure layer 120, but the grating structure layer 120 can serve as acladding layer formed to confine light. The substrate 110 constitutingthe quantum dot diode 100 is an InP substrate or a GaAs substrate.

Referring to FIG. 1B, a first lattice-matched layer 140 is formed on thegrating structure layer 120. The first lattice-matched layer 140 is alayer for growing In(Ga)As quantum dots using a self-assembly method onthe InP substrate 110 on which the grating structure layer 120 isformed. The first lattice-matched layer may be formed bylattice-matching InAlGaAs and In(Ga,As)P. The first lattice-matchedlayer 140 has a heterojunction structure formed by stacking at least twomaterial layers formed of a group III-V compound, e.g., In, Al, Ga, etc.in group III, and As, P, etc. in group V, and serves as a barrier layer.In addition, the first lattice-matched layer 140 may be formed to have aseparate confinement hetero (SCH) structure having a step index(SPIN)-shaped waveguide or a graded index (GRIN)-shaped waveguide.

Referring to FIG. 1C, a quantum dot layer 150 consisting of In(Ga)Asquantum dots is formed on the first lattice-matched layer 140. Thequantum dot layer 150 consists of the In(Ga)As quantum dots obtained bygrowing In(Ga)As using the self-assembly mechanism, and the quantum dotsmay be grown by one of a metal-organic chemical vapor deposition (MOCVD)method, a molecular beam epitaxy (MBE) method, an atomic layer epitaxy(ALE) method and a chemical beam epitaxy (CBE) method. In thisembodiment, the quantum dots constituting the quantum dot layer are notspaced apart from each other but may be formed at predeterminedintervals in a quantum dot structure.

Meanwhile, although not disclosed in this embodiment and drawings, whenthe first lattice-matched layer 140 is formed to have a SPIN-shaped SCHstructure, quantum dots in a quantum well (DWELL) formed by insertingquantum wells in the SPIN SCH structure may be symmetrically orasymmetrically surrounded by the quantum wells. In the same manner, whenthe first lattice-matched layer 140 is formed to have a GRIN-shaped SCHstructure, the quantum dots may be symmetrically or asymmetricallysurrounded by quantum wells inserted into the GRIN SCH structure. Inthis embodiment, one quantum dot layer is formed, but a plurality ofquantum dot layers having a plurality of quantum dots may be stacked.

Referring to FIG. 1D, after the quantum dot layer 150 including aplurality of quantum dots is formed, a second lattice-matched layer 141is formed on the quantum dot layer 150. The second lattice-matched layer141 is the same as the first lattice-matched layer 140 in structure andfunction and also may be formed into a barrier layer or to have an SCHstructure.

Referring to FIG. 1E, after the second lattice-matched layer 141 isformed on the quantum dot layer 150 having the quantum dot structure, acladding layer 160 capable of serving to confine light emitted from thequantum dot layer 150 is formed. The cladding layer 160 may be formed ofInAl(Ga)As or In(Ga,As)P.

Referring to FIG. 1F, an ohmic contact layer 170 is formed on thecladding layer 160. The ohmic contact layer 170 is for adjusting anohmic contact and formed of InGaAs in this embodiment.

FIG. 2A is a cross-sectional image of a quantum dot laser diode taken bya scanning electron microscope (SEM) according to an exemplaryembodiment of the present invention, and FIG. 2B is an image of asurface of a quantum dot laser diode taken by an optical microscopeaccording to an exemplary embodiment of the present invention.

Referring to FIG. 2A, the quantum dot laser diode according to anexemplary embodiment of the present invention includes a gratingstructure layer 120 including gratings 130, and a quantum dot layer 150including In(Ga)As quantum dots and formed on the grating structurelayer 120. Referring to FIG. 2B, a quantum dot laser diode is shownwhich is manufactured by growing a distributed feedback laser diodeusing the quantum dot layer 150 formed on a grating structure layer andhaving an In(Ga)As quantum dot structure and InAlGaAs—InAlAs materialseries and then forming it in a ridge-waveguide shape A.

FIG. 3 illustrates light emission spectrum graph (i) of a quantum dotlaser diode including a grating structure layer and light emissionspectrum graph (ii) of a quantum dot laser diode not including a gratingstructure layer according to an exemplary embodiment of the presentinvention. Light emission spectrum experiments were performed at roomtemperature under the same condition, e.g., a quantum dot layer growthcondition, except that gratings are included (i) or not (ii).

Referring to FIG. 3, the x-axis indicates wavelength (μm), and they-axis indicates intensity (arb. units). Graph (i) is a light emissionspectrum graph of a quantum dot laser diode in which a grating structurelayer is included and a 7 quantum dot layer is stacked on the gratingstructure layer. And, graph (ii) is a light emission spectrum graph of aquantum dot laser diode in which a grating structure layer is notincluded and a 7 quantum dot layer is stacked. From graphs (i) and (ii),it can be seen that the quantum dot laser diode including gratings canprovide higher gain than the quantum dot laser diode not includinggratings in a desired wavelength region of about 1.55 μm or above. Theabove experiments used 7 quantum dots but are not limited hereto.

FIG. 4A is a graph showing optical power as a function of appliedcurrent measured under a continuous wave (cw) condition at roomtemperature in a quantum dot laser diode manufactured according to anexemplary embodiment of the present invention, and FIG. 4B is a graphshowing a spectrum obtained by applying a predetermined current to aquantum dot laser diode manufactured according to an exemplaryembodiment of the present invention.

Referring to FIG. 4A, the variation of optical power as a function ofapplied current under a continuous wave condition at room temperature ina quantum dot laser diode according to an exemplary embodiment of thepresent invention is shown. The x-axis indicates applied current (I:mA),and the y-axis indicates optical power (power:mW). The graph of FIG. 4Aillustrates the result of an experiment using a ridge-waveguide-shapedquantum dot laser diode including a quantum dot layer having In(Ga)Asquantum dots and manufactured according to an exemplary embodiment ofthe present invention. A ridge-waveguide used in this experiment had aridge width of 3 μm. The ridge-waveguide had been cut off to make acavity length of 1 mm, and then the experiment was performed under thecontinuous wave condition at room temperature. According to the resultof the experiment, optical power did not vary until current applied tothe quantum dot laser diode became a predetermined value but wasproportional to the applied current when the applied current becameabout 40 mA or more. In result, it can be seen that optical power of thequantum dot laser diode according to an exemplary embodiment of thepresent invention is proportional to applied current, stable anduniform.

Referring to FIG. 4B, the spectrum of a quantum dot laser diode measuredunder the same condition of FIG. 4A according to an exemplary embodimentof the present invention is shown. In FIG. 4B, the x-axis indicateswavelength (μm), and the y-axis indicates intensity (dB). From thespectrum of FIG. 4B, it can be seen that relatively high gain can beobtained at a desired emission wavelength. In this embodiment, anextraordinary intensity of about −15 dB is shown at a wavelength ofabout 1.564 μm in comparison with other wavelength. The value is higherthan that of any other wavelength by about 42 dB. Consequently, when aquantum dot laser diode according to an exemplary embodiment of thepresent invention is used, relatively high gain can be obtained at adesired wavelength and device characteristics can be improved.

As described above, it is possible to provide a quantum dot laser diodeshowing excellent gain at a desired wavelength without affecting quantumdot uniformity by forming a grating structure under quantum dots, i.e.,adopting gratings (structure) as a variable affecting a quantum dotactive layer.

While the invention has been shown and described with reference tocertain exemplary embodiments thereof, it will be understood by thoseskilled in the art that various changes in form and details may be madetherein without departing from the spirit and scope of the invention asdefined by the appended claims.

1. A quantum dot laser diode, comprising: a grating structure layerformed on a substrate and including a plurality of gratings; a firstlattice-matched layer formed on the grating structure layer; at leastone quantum dot layer formed on the first lattice-matched layer andincluding at least one quantum dot; a second lattice-matched layerformed on the quantum dot layer; a cladding layer formed on the secondlattice-matched layer; and an ohmic contact layer formed on the claddinglayer.
 2. The quantum dot laser diode of claim 1, wherein the substrateis one of an InP substrate and a GaAs substrate.
 3. The quantum dotlaser diode of claim 1, wherein the quantum dot is formed of In(Ga)As.4. The quantum dot laser diode of claim 1, wherein the first and secondlattice-matched layers are formed of a group III-V compound.
 5. Thequantum dot laser diode of claim 1, wherein the first and secondlattice-matched layers have a separate confinement hetero (SCH)structure having a SPIN or GRIN-shaped waveguide.